Flash evaporation (or partial evaporation) is the partial vapor that occurs when a
saturated liquid
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
**Degree of unsaturation
**Saturated fat or fatty acid ...
stream undergoes a reduction in pressure by passing through a
throttling valve or other throttling device. This process is one of the simplest
unit operation
In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomeriza ...
s. If the throttling valve or device is located at the entry into a
pressure vessel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a
flash drum.
If the saturated liquid is a single-component liquid (for example,
propane
Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
or liquid
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
), a part of the liquid immediately "flashes" into vapor. Both the vapor and the residual liquid are cooled to the
saturation temperature
The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The boiling point of a liquid varies depending upon the surrounding envir ...
of the liquid at the reduced pressure. This is often referred to as "auto-refrigeration" and is the basis of most conventional
vapor compression refrigeration
Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase transition, phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning ...
systems.
If the saturated liquid is a multi-component liquid (for example, a mixture of
propane
Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
,
isobutane and normal
butane
Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name but ...
), the flashed vapor is richer in the more
volatile components than is the remaining liquid.
Uncontrolled flash evaporation can result in a boiling liquid expanding vapor explosion (
BLEVE
A boiling liquid expanding vapor explosion (BLEVE, ) is an explosion caused by the rupture of a vessel containing a pressurized liquid that has reached temperature above its boiling point. Because the boiling point of a liquid rises wi ...
).
Flash evaporation of a single-component liquid
The flash evaporation of a single-component liquid is an
isenthalpic
An isenthalpic process or isoenthalpic process is a process that proceeds without any change in enthalpy, ''H''; or specific enthalpy, ''h''.
Overview
If a steady-state, steady-flow process is analysed using a control volume, everything outside t ...
process and is often referred to as an
adiabatic flash. The following equation, derived from a simple heat balance around the throttling valve or device, is used to predict how much of a single-component liquid is vaporized.
:
:
If the enthalpy data required for the above equation is unavailable, then the following equation may be used.
:
:
Here, the words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.
This type of flash evaporation is used in the
desalination
Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in Soil salinity control, soil desalination, which is an issue f ...
of
brackish
Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estu ...
water or ocean water by "
Multi-Stage Flash
Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Current MSF facilities may hav ...
Distillation." The water is heated and then routed into a reduced-pressure flash evaporation "stage" where some of the water flashes into steam. This steam is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure. More water is flashed into steam which is also subsequently condensed into more salt-free water. This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are met. A large part of the world's installed desalination capacity uses multi-stage flash distillation. Typically such plants have 24 or more sequential stages of flash evaporation.
Equilibrium flash of a multi-component liquid
The equilibrium flash of a multi-component liquid may be visualized as a simple
distillation
Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distilla ...
process using a single
equilibrium stage A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Such equilibrium stages may also be referred to as a ...
. It is very different and more complex than the flash evaporation of single-component liquid. For a multi-component liquid, calculating the amounts of flashed vapor and residual liquid in equilibrium with each other at a given temperature and pressure requires a trial-and-error
iterative
Iteration is the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. ...
solution. Such a calculation is commonly referred to as an equilibrium flash calculation. It involves solving the ''Rachford-Rice equation'':
Flash Calculations using the Soave-Redlich-Kwong equation of state
(view full-size image)[Curtis H. Whitson, Michael L. Michelsen, ''The Negative Flash'', Fluid Phase Equilibria, 53 (1989) 51–71.]
:
where:
* ''zi'' is the mole fraction of component ''i'' in the feed liquid (assumed to be known);
* ''β'' is the fraction of feed that is vaporised;
* ''Ki'' is the equilibrium constant of component ''i''.
The equilibrium constants ''Ki'' are in general functions of many parameters, though the most important is arguably temperature; they are defined as:
:
where:
* ''xi'' is the mole fraction of component ''i'' in liquid phase;
* ''yi'' is the mole fraction of component ''i'' in gas phase.
Once the Rachford-Rice equation has been solved for ''β'', the compositions ''xi'' and ''yi'' can be immediately calculated as:
:
The Rachford-Rice equation can have multiple solutions for ''β'', at most one of which guarantees that all ''xi'' and ''yi'' will be positive. In particular, if there is only one ''β'' for which:
:
then that ''β'' is the solution; if there are multiple such ''βs, it means that either ''K''max<1 or ''K''min>1, indicating respectively that no gas phase can be sustained (and therefore ''β''=0) or conversely that no liquid phase can exist (and therefore ''β''=1).
It is possible to use Newton's method
In numerical analysis, Newton's method, also known as the Newton–Raphson method, named after Isaac Newton and Joseph Raphson, is a root-finding algorithm which produces successively better approximations to the roots (or zeroes) of a real-valu ...
for solving the above water equation, but there is a risk of converging to the wrong value of ''β''; it is important to initialise the solver to a sensible initial value, such as (''βmax''+''βmin'')/2 (which is however not sufficient: Newton's method makes no guarantees on stability), or, alternatively, use a bracketing solver such as the bisection method
In mathematics, the bisection method is a root-finding method that applies to any continuous function for which one knows two values with opposite signs. The method consists of repeatedly bisecting the interval defined by these values and the ...
or the Brent method
In numerical analysis, Brent's method is a hybrid root-finding algorithm combining the bisection method, the secant method and inverse quadratic interpolation. It has the reliability of bisection but it can be as quick as some of the less-reliable ...
, which are guaranteed to converge but can be slower.
The equilibrium flash of multi-component liquids is very widely utilized in petroleum refineries
An oil refinery or petroleum refinery is an industrial process plant where petroleum (crude oil) is transformed and refined into useful products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, liquefie ...
, petrochemical
Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sou ...
and chemical plant
A chemical plant is an industrial process plant that manufactures (or otherwise processes) chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transform ...
s and natural gas processing plants.
Contrast with spray drying
Spray drying
Spray drying is a method of changing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals, or materials which may requ ...
is sometimes seen as a form of flash evaporation. However, although it is a form of liquid evaporation, it is quite different from flash evaporation.
In spray drying, a slurry of very small solids is rapidly dried by suspension in a hot gas. The slurry is first atomized into very small liquid droplets which are then sprayed into a stream of hot dry air. The liquid rapidly evaporates leaving behind dry powder or dry solid granules. The dry powder or solid granules are recovered from the exhaust air by using cyclones
In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
, bag filters or electrostatic precipitator
An electrostatic precipitator (ESP) is a filterless device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.
In con ...
s.
Natural flash evaporation
Natural flash vaporization or flash deposition may occur during earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s resulting in deposition of minerals held in supersaturated solution
In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a liqu ...
s, sometimes even valuable ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
in the case of auriferous, gold-bearing, waters. This results when blocks of rock are rapidly pulled and pushed away from each other by jog faults.
See also
*Evaporator
An evaporator is a device used to turn the liquid form of a chemical substance, such as water, into a vapor.
Uses
Air conditioning and refrigeration
Some air conditioners and refrigerators use a compressed liquid with a low boiling point, su ...
* Vapor-liquid separator
*Multi-stage flash distillation
Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Current MSF facilities may ha ...
References
External links
Vapor and Flash Steam
Animation, photos and technical explanation of the difference between Flash Steam and Vaporized fraction.
Flash Steam Tutorial
The benefits of recovering flash steam, how it is done and typical applications.
Water Desalination Technologies
in the Middle East and Western Asia
Discussion of spray drying
Flash distillation of the hydrocarbon compounds.
{{States of matter
Gas-liquid separation
Evaporators